The present invention generally relates to pulmonary ventilation and, more particularly, to a pulmonary ventilator using a "breathable liquid".
As used herein, the phrase "breathable liquid" is meant to refer to liquids which are capable of delivering oxygen to, and removing carbon dioxide from, the pulmonary system or lungs of a patient. Examples of "breathable liquids" include, but are not limited to, saline, silicone, vegetable oils, perfluorochemicals and others. One of the presently preferred "breathable liquids" is perfluorocarbon liquid. Hereinafter, the phrase "perfluorocarbon liquid" is used in place of "breathable liquids". This use, however, is not intended to restrict the present invention specifically thereto.
Perfluorocarbon liquids, also known as perfluorocarbons or simply PFC's, are derived from common organic compounds in which fluorine atoms have replaced carbon bound hydrogen atoms. PFC's are colorless, odorless, non-flammable liquids which have a high dielectric strength and resistivity. They are substantially insoluble in water, are denser than water, exhibit low surface tensions, and have low viscosities. Perhaps the most unique characteristic of PFC's is their high affinity for gases, dissolving up to twenty times as much oxygen and over three times as much carbon dioxide as water. Like other materials which are widely used in the practice of medicine, PFC's are extremely biocompatible and non-toxic. In addition to liquid ventilation, perfluorocarbon liquids have found utility as artificial blood substitutes, in lung lavage (washing) and medical treatments such as convective lung hypothermia.
Pulmonary ventilation with various breathable liquids has been investigated from time to time during the last three decades and the ability to provide for adequate oxygenation and the elimination of carbon dioxide during liquid ventilation has been demonstrated. It was first demonstrated in the early 1960's that mammals could be submerged in hyperoxygenated saline, could breath the oxygenated liquid and could successfully resume gas breathing thereafter. Because of the practical difficulties involved with dissolving sufficient quantities of oxygen in saline (high pressures are required) and because the saline rinsed away much of the surfactant lining in the lung alveoli, the saline approach to liquid ventilation was eventually abandoned. These problems were overcome in the mid 1960's when PFC's were first used to support the respiration of various small animals. The biocompatibility and various other properties of certain PFC's has led to a significant body of ongoing research which appears to support promising clinical applications, including the treatment of Respiratory Distress Syndrome (RDS), Adult Respiratory Distress Syndrome (ADRS) and other situations where surfactant deficiencies in the lung compromise pulmonary functions.
To date, it has been clearly established that oxygenated perfluorocarbons can be used to provide total ventilation support over an extended period of time while enabling a return to gas breathing without long term side effects. The side effects which do result are typically minor and transient in nature (mild acidosis, lower blood CO.sub.2, increased pulmonary vascular resistance and decreased lung compliance). Additionally, the various studies have also shown that PFC ventilation results in no adverse morphological, biochemical or histological effects.
In the United States, liquid ventilation has only been clinically studied in a few instances. Generally, these systems have been gravity systems where, during inspiration, oxygenated perfluorocarbon is placed into a reservoir and allowed to drain into the patient's lungs under the influence of gravity. During expiration, another reservoir is placed below the level of the patient's lungs and the perfluorocarbon is allowed to drain from the lungs, again under the influence of gravity.
In the laboratory, complex liquid ventilation systems utilizing modern extracorporeal life support technology have been described and used. Specifically, these systems have required the use of multiple roller pumps and multiple fluid reservoirs in order to provide control over all the parameters of inspiration, expiration, oxygenation and carbon dioxide removal. These systems have also necessitated the use multiple loop or bypass circuits for the perfluorocarbon ensure adequate inspiration and expiration. The systems have also included, in addition to the bypass circuit, the use of multiple valves to properly direct the perfluorocarbon into the lungs during inspiration and through the bypass circuit during expiration. The multiple valve and bypass systems have typically included a valve located in the circuit both before and after the patient, on what are known as the inspiratory limbs and expiratory limbs of the circuit. The valves are manipulated during inspiration and expiration so that the perfluorocarbon is either directed into the lungs (during inspiration) or diverted through the bypass circuit.
As the above discussion reveals, these prior liquid ventilation systems are quite complex and, often, too cumbersome to make clinical application practical. The need for a simplified and reliable liquid ventilation system is therefore obvious.
In achieving its main objective, namely, providing positive pressure, tidal volume liquid ventilation, the present invention overcomes and substantially alleviates the problems associated with the known prior systems.
Another object of the present invention is to provide a liquid ventilator which reduces the mechanical complexity of a liquid ventilation system through the adoption of a more simplified design. A related object is to allow both inspiration and expiration while eliminating the need for a bypass circuit.
Still another object of this invention is to provide a liquid ventilation system in which the breathable liquid is continuously being moved or pumped through the system.
In achieving the above and other objects, the present invention provides a pulmonary ventilator in which the breathable liquid, preferably perfluorocarbon, is recirculated through a single, closed loop circuit while adequately providing for both inspiration and expiration. The circuit of the present invention generally contains the following components connected in series by a conduit: a reservoir, a pump, an oxygenator, a heat exchanger, a Y-connector, an endotracheal tube, a valve and a controller.
The conduit leads from the reservoir to a continuously operable pump which forces or drives the perfluorocarbon through the circuit. From the pump, the perfluorocarbon travels to the oxygenator where carbon dioxide is removed and the perfluorocarbon is oxygenated. The perfluorocarbon is transferred from the oxygenator to a heat exchanger where it is heated or cooled to the desired temperature. From the heat exchanger, the perfluorocarbon travels through the conduit to the inspiratory limb of the Y-connector. The other two limbs of the Y-connector are an expiratory limb and an endotracheal limb. The endotracheal limb is attached to the endotracheal tube which extends into the lungs of the patient. The expiratory limb is connected by the conduit to the valve mentioned above. From the valve, the conduit feeds the perfluorocarbon back into the reservoir.
During operation, to simulate inspiration by the patient, the controller causes the valve to close thereby forcing the oxygenated and heated perfluorocarbon being driven by the pump through the inspiratory limb of the Y-connector, out the endotracheal limb and tube into the lungs of the patient. After a tidal volume of perfluorocarbon has been pumped into the lungs, as determined by the flow rate of the perfluorocarbon, the controller opens the valve. The perfluorocarbon within the patient's lungs is then entrained from the lungs as a result of the Y-connector. The specific configuration of the Y-connector generates a venturi effect which enhances drainage of the perfluorocarbon from the lungs while allowing for continuous profusion of the perfluorocarbon through the circuit. Passing through the valve, the carbon dioxide laden perfluorocarbon is directed back into the reservoir where the ventilation process begins to repeat its cycle.
The present invention is advantageous in a clinical setting since it provides a simple, single circuit liquid ventilator which is capable of continuously perfusing the perfluorocarbon without the need for an additional bypass circuit or multiple valves. Since the present invention requires only one valve, improvements in terms of mechanical simplicity and safety are achieved. While continued laboratory studies of present liquid ventilation system are ongoing, the data so far generated has demonstrated marked improvements in pulmonary function and gas exchange when compared to gas ventilation in animals with severe respiratory failure. It is therefore anticipated that the present invention will play a significant role in providing positive, tidal volume liquid ventilation during the treatment of severe respiratory failure in a simple yet safe manner.
Additional benefits and advantages of the present invention will become apparent to those skilled in art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.